The enhanced transmission method and system proposed in the present invention could be used in any network using 2G or 3G Technology, 2G LTE (Long Term Evolution), WIMAX, HSDPA technology, and generally speaking to any wireless transmission technology. It is especially useful in mobile networks systems using Multiple Input-Multiple Output, MIMO, technique
Multiple-input Multiple-output (MIMO) techniques are well known and they are used in wireless networks, including 3G Mobile networks.
In MIMO systems, both the transmitter and the receiver are equipped with multiple antennas in order to improve the system performance. In particular, the use of MIMO systems represents a useful solution for improving the capacity and user throughput performance of the networks.
The basic MIMO feature as standardised in 3GPP Release 7 is based on two transmitter antennas (at the node B) and two receiving antennas (at the UE) using a single carrier. At the transmitter, the data can be split into one or two data streams and transmitted through the two antennas using the same radio resource (i.e. same transmission time interval and HSDPA codes). In a generic downlink transmitter structure to support MIMO operation the primary and secondary transport blocks are each processed (channel coding and interleaving), then spread, subsequently weighted by precoding weights. Finally, the resulting channels after MIMO precoding (i.e. MIMO channel#1 and MIMO channel#2) are mapped onto P-CPICH and S-CPICH (Primary and secondary common pilot channels), respectively before being provided to the first and second physical antennas respectively.
The two streams of data are recovered by the UE from the signals received via its two antennas (Rx Diversity). Thus, for the MIMO feature to work both the network and the terminals need to be MIMO-enabled. In order to deploy MIMO and transmit two parallel data streams, two power amplifiers are required per sector (one for each of the two antennas. In order not to use an entire carrier for MIMO only (5 MHz), it is more efficient and practical to use the same carrier for MIMO devices as used for non-MIMO devices (e.g. HSDPA legacy terminals).
Another technique frequently used to improve the performance of 3G wireless networks is the High-Speed Downlink Packet Access HSDPA technology. HSDPA is a packet-based data service in third generation (3G) W-CDMA (Wideband Code Division Multiple Access) systems, which provides high-speed data transmission (with different download rates e.g. 7.2/10.8/16.2/21.6 Mbps over a 5 MHz bandwidth) to support multimedia services.
HSDPA comprises various versions with different data speeds and features. The following table is derived from table 5.1a of the release 9 version of 3GPP TS 25.306 and shows maximum speeds of different device classes and the combination of features they support.
TABLE 1HS-DSCH physical layer categoriesMaximumSupportednumber ofmodulationsbits of anSupportedsimultaneousSupportedMaximumHS-DSCHTotalmodulationswith MIMOSupportedmodulationsnumberMax.transport blocknumberwithout MIMOoperationmodulationssimultaneousof HS-DSCHdatareceivedof softoperation orand withoutwith dualdual cellHS-DSCHcodesratewithin anchanneldual celldual cellcelland MIMOcategoryreceived[Mbit/s]HS-DSCH TTIbitsoperationoperationoperationoperationCategory 151.2729819200QPSK,NotNotNotCategory 251.272982880016QAMapplicableapplicableapplicableCategory 351.8729828800(MIMO(dual cell(simultaneousCategory 451.8729838400notoperationdual cellCategory 553.6729857600supported)notand MIMOCategory 653.6729867200supported)operationCategory 7107.214411115200notCategory 8107.214411134400supported)Category 91510.120251172800Category 101514.027952172800Category 1150.9363014400QPSKCategory 1251.8363028800Category 131517.635280259200QPSK,Category 141521.14219225920016QAM,64QAMCategory 151523.423370345600QPSK, 16QAMCategory 161528.027952345600Category 1715—35280259200QPSK,—16QAM,64QAM23370345600—QPSK,16QAMCategory 1815—42192259200QPSK,—16QAM,64QAM27952345600—QPSK,16QAMCategory 191535.335280518400QPSK, 16QAM, 64QAMCategory 201528.042192518400Category 211542.223370345600——QPSK,Category 221542.22795234560016QAMCategory 231523.435280518400QPSK,Category 241528.04219251840016QAM,64QAMCategory 251535.323370691200———QPSK,16QAMCategory 261542.227952691200Category 271546.7352801036800———QPSK,Category 281555.942192103680016QAM,64QAM
In order to reach yet higher peak rates (i.e. 28.8 Mbps with 3GPP Release 7), the MIMO (Multiple Input Multiple Output) feature is used in HSDPA. MIMO technology is an important step in the evolution of HSDPA, as it provides higher data rates in downlink whilst further improving spectrum efficiency.
When introducing MIMO in a system, it is indispensable to have two transmission branches (RF chains), including two power amplifiers each one connected to the physical antenna. In order to optimise the usage of the power resource it is highly desirable to balance the power between the two power amplifiers. Whilst MIMO channels are intrinsically perfectly power balanced, all the remaining channels need to be transmitted with equal power by each power amplifier. To this end, two techniques can be used: a first one is the use of transmission diversity (using “Space Time Transmit Diversity” (STTD) for all non-MIMO channels except for the Synchronisation Channel for which “Time Switch Transmit Diversity” (TSTD) is used). Another technique is referred to as Virtual Antenna Mapping (VAM) in this description and is discussed herein after.
Another key requirement is to make sure that the technique used to power balance the non-MIMO channels allows the same performance as would be achieved with the same energy using a single power amplifier. STTD was defined by 3GPP (Release '99) in order to achieve this. However in practice this feature has been found to affect the performance of certain legacy user equipments. In particular HSDPA UEs with equalizer receivers can be severely impacted. This is due to the time transformation which is performed by STTD, which is ill-adapted to an optimum equalization process. Some HSDPA devices have been found to deactivate their equalizer for this reason. Field tests have shown that the impact of the use of STTD on the throughput of data received by an HSDPA category 8 device (especially for a type 2 receiver i.e. a single antenna equalizer receiver) is particularly negative under good and medium radio conditions.
Virtual Antenna Mapping is an alternative which is aimed at solving this issue fulfilling both above-mentioned requirements. Hence, this technique enables power balancing of the power amplifiers whilst not impacting on the performance of legacy users. The principle of the VAM technique is depicted in FIG. 1. The VAM operation/function 100 can be performed as a baseband function after the mapping onto physical channels for Rel'99 and HSDPA and after precoding for MIMO. The VAM operation/function can also be implemented in logic in a radio unit such as a Remote Radio Head (RRH). The signals shown at the input of adding operations 150 are the following: Rel'99 refers to the dedicated channel (DCH) which can carry voice or data traffic. It refers to HSDPA SIMO (Single Input Multiple Output, i.e. HSDPA without MIMO). MIMO Channel #1 is the resulting channel after MIMO precoding operation as can be seen in FIG. 11 consisting of the sum of the primary data stream and the secondary data stream weighted with their correspondent weights, and MIMO Channel #2 is the resulting channel after MIMO precoding operation as can be seen in FIG. 11 consisting of the sum of the primary data stream and the secondary data stream with their correspondent weights. VAM consists of mapping input signals onto the physical antennas with specific weights for each path. VAM can be seen as a matrix of four weights ω1, ω2, ω3, ω4 and two adders 110 applied to two input signals fed by “virtual antennas” 160, 170 corresponding to the physical antennas depicted in FIG. 1, showing the MIMO operation. The force of the virtual antenna concept is that the UE behaves as if the signals present at the virtual antennas are the ones actually transmitted, although the physical antennas radiate something different. The legacy UE (not supporting MIMO) will only see the virtual antenna 160. Whilst its signal will be transmitted on both physical antennas the UE receiver will act as if transmitted from one (the mapping between virtual and physical antennas is transparent for the user equipment). The configuration received by the legacy user is the same as in a single antenna transmission system, the user equipment is not configured for any form of transmit diversity at RRC level. The MIMO UE will see both virtual antenna 160 and virtual antenna 170 and is unaware of the mapping between the virtual and physical antennas, which is transparent to the MIMO operation.
The four weights from the VAM matrix are differentiated by phases only as equal amplitude is required to achieve power balancing between the two physical antennas 120,130. A first power amplifier 140 and a second power amplifier 150 are configured for amplifying the output signals after the VAM function before they are radiated by the physical antennas 120,130. The weights of the VAM matrix are fixed. They are configured for the whole cell and set by Operation & Maintenance (O&M) and typically not changed very often. The VAM weights fulfil totally different objectives than the MIMO precoding weights—the latter ones being variable weights (that can change every 2 ms) used only for the purpose of the MIMO transmission whilst VAM applies to all channels and has as objective to fulfil the two requirements highlighted above.
From the legacy user point of view the VAM technique is like a single antenna transmission, i.e. the user terminal demodulates the HSDPA signal as if there were no Transmission diversity in the system. Seen from the transmit side for legacy non-MIMO users, VAM amounts to transmitting the same signal (common channel, Rel'99 and HSDPA non-MIMO) on the two transmit antenna ports but with a different phase.
However, from extensive field testing of VAM functionality (measurements over a large amount of static points which statistically shows the impact of VAM), the following results have been obtained:                When there is no concurrent HSDPA and active MIMO user equipments e.g. only HSDPA (non-MIMO) user equipments in the cell, VAM has little or no impact on HSDPA performance i.e. throughputs observed of HSDPA user equipments with VAM active are nearly the same as the throughputs of HSDPA without VAM (single antenna transmission as in most 3G networks today).        The performance of MIMO with VAM is also very similar to the performance of MIMO with Tx diversity (STTD).        However whenever there is concurrent HSDPA and MIMO traffic, it has been observed that the performance of HSDPA legacy devices is impacted negatively by around 10% for a legacy type 3 device (Rx diversity and equalizer implemented in receiver) and by around 15-20% for a legacy type 2 HSDPA device (no Rx diversity, only equalizer implemented in receiver) whenever the secondary pilot is present in the second virtual antenna and more degradation is observed whenever the MIMO user is fully active with continuous downloads.        
Hence, it is shown that even though the VAM technique has a better performance than previously used techniques such as STTD, it has still a negative impact in HSDPA legacy devices when there is concurrent HSDPA and MIMO traffic.
There is therefore a need in the art for transmission schemes which further improve the performance of legacy HSDPA devices in concurrent HSDPA-MIMO traffic while maintaining the advantages of using VAM techniques.